The invention is in an improved method and apparatus for the production of benzenecarboxylic acids and benzenedicarboxylic acid esters or mixture thereof by the oxidation of xylenes or toluylic acid esters by means of oxygen or gases containing oxygen, in the presence of catalysts containing heavy metal, at elevated temperatures and elevated pressure.
Terephthalic acid dimethyl esters can be prepared by oxidizing p-xylene (PX) with air in the liquid phase. In a first phase of the preparation, a methyl group of the PX is oxidized in the presence of a heavy metal catalyst to form p-toluylic acid. The acid is then esterified with methanol to p-toluylic acid methyl ester (PT-ester=PTE). PTE is then further oxidized together with PX to the terephthalic acid monomethyl ester, and is then transformed with methanol to terephthalic acid dimethyl ester end product which is separated off by distillation, while the remaining part is the so called working ester.
The two oxidation processes are performed together, insofar as possible. The liquid reaction phase placed in the reactor is a mixture preferably prepared from fresh PX and the working ester (derived from the esterification with a high content of PTE plus an amount of by-products) the amount of the working ester being the predominate portion. This reaction phase used as starting mixture allows operation in the fluid state for all of the substances to be manipulated during the production process.
The progress of the oxidation of the starting mixture to form the oxidator liquid which contains specified contents of the target products is determined by measuring the acid number of the reaction phase in the reactor which is called the "oxidator". The acid number is measured in milligrams of potassium hydroxide per gram of the oxidator liquid. By the acid number only the carboxylic acid group-COOH is measured. The selectivity of the oxidation is determined by analyzing the reactor exhaust gas in which a content of carbon monoxide and carbon dioxide indicates an undesired decarboxylation and the content of residual oxygen indicates the utilization grade of the oxygen.
Production on a large technical scale is performed continuously in the conventional process with a pressure between 0.5 and 1.1 MPa at a temperature of from 135.degree. to 170.degree. C. An effort is made to operate with a high air input or high yields per unit time and capacity, at a high oxygen reaction and a low yield of carbon monoxide and carbon dioxide and by-product formation, and at the same time to carry off the considerable reaction heat in a narrow temperature range in the oxidation zone adapted to the progress of the reaction. The oxidators are constructed, for example, in the form of bubble column reactors of 10 or more meters in height. The reaction gas is introduced through gas distributors in the vertical liquid column.
Limitations are created by frothing and oxygen gas breakthroughs which lower operating safety and economy. The reaction heat is carried away and recovered by evaporating boiler feed water in heat exchanging systems inside or outside of the oxidator. Highly developed but very expensive tubular reactors are standard equipment.
In another procedure the oxidation gas is introduced into the oxidator liquid by gas jets, so as to achieve high substance transformations, the reaction heat being removed as described above. In this procedure too, the amount of air introduced also is limited by elevated oxygen contents and combustion products in the exhaust gas, as well as unavoidable frothing in the oxidator. Very high or very long oxidators are required.
Due to the highly exothermic oxidation reaction, local temperature peaks are unavoidable in either process. This results in an elevated formation of CO, CO.sub.2, increased amounts of low aliphatic acids and oxidation products of high molecular weight, all of which result in yield losses.
The objective therefore was to increase the selectivity and yield of the reaction, i.e., the formation of terephthalic acid monomethyl esters from PTE and of p-toluylic acid from PX as well as to increase the rate of oxidation, and by reducing the residence time of the liquid products in the oxidator to reduce the thermal stress on the products, and to manage i.e. control the increased amount of heat per unit time resulting therefrom, while maintaining favorable reaction temperatures and conditions.